Trackless tugger trains can be used, in particular, as industrial trucks for transportation within a company's premises, for example. A tugger train usually has at least one transportation module, preferably two or more transportation modules which are arranged one behind the other and which can be embodied in such a way that they can transport loads or goods, in particular horizontally. In addition, the tugger train generally has a towing vehicle which is harnessed in front of the first transportation module when viewed in the direction of travel, in order to tow the transportation modules and therefore move in a desired direction. An important objective when configuring a tugger train is that the transportation modules follow the towing vehicle with the greatest possible directional stability. Small directional deviations of the transportation modules, or even none at all if possible, permit narrow travel lanes of the tugger train, with the result that the tugger train can be moved safely even on small traffic areas. Furthermore, the directional stability reduces the risk of collision accidents, since transportation modules which do not move in the track of the towing vehicle can quickly collide in bends with persons or objects located at the edge of the travel lane.
Various configurations of the running gear units of the transportation modules are known from the prior art. For example, the transportation modules can have two main wheels which are arranged on a wheel axle in a non-pivotable fashion. In addition to the main wheels, further support rollers, which are embodied as trailing rollers, can preferably be arranged at the corners of the transportation module in order to safely support the transportation module. These support rollers can orient themselves automatically and therefore usually do not influence the trailing behaviour and the directional stability significantly. The running gear unit of these two-wheel or single axle transportation modules is of simple design and can therefore be manufactured cost-effectively. Since no device is required to steer the wheels, the running gear unit has a low level of complexity, as a result of which it requires little maintenance and is operationally reliable. The disadvantage of this running gear unit which is arranged directly on the transportation module is, however, that the axle with the two main wheels has to be located approximately in the centre of the transportation module in order to be able to achieve a good trailing behaviour and therefore a high level of conventional stability of the transportation module. However, this considerably restricts the possibilities in terms of loading the transportation modules with rollable transportation frames. In particular, unless additional technical aids are used, transportation modules with such running gear units cannot be loaded on both sides with relatively large transportation frames whose footprints dimensions correspond approximately to the width of the transportation modules.
In order to be able to permit the transportation modules to be loaded and unloaded on both sides with transportation frames or other rollable cargo carriers, four-wheeled transportation module running gear units have been developed for a tugger train in which in each case two wheels are arranged on one axle. These wheels are mounted in such a way that they can be pivoted about a vertical axis using a mechanical or hydraulic steering device. A trackless tugger train with such a running gear unit is known, for example, from DE 10 2008 060 801 B3. Steering angles of the wheels are changed here by the steering device as a function of the angle between the trailer drawbar and the trailer longitudinal axis. The individual steering devices are coupled to one another here, for example, via a linkage, with the result that the steering angles of the front wheels correspond in value to the steering angles of the rear wheels. It is disadvantageous here that the steering movement of the front axle wheels has to be transmitted to the rear wheels by reversing the direction. This is difficult, in particular, when the transportation modules are to be loaded and unloaded on both sides with rollable transportation frames. For this purpose, an embodiment of the transportation modules is used which is referred to as a U-frame and whose supporting frame has, when viewed from the side, the shape of a gantry which is open at the bottom. The pillars of this gantry are supported here by the two wheel axles. The transmission of the steering lock from the front axle to the rear axle is particularly costly here, since the movement has to be transferred over relatively large distances and experiences a plurality of directional deflections. The technical expenditure on implementation for such a tugger train is therefore very high, as a result of which both the manufacturing costs and the maintenance requirements are high.